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Nuclear Energy Fundamentals

Module 3: Nuclear Reactor Types

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Academic Services

April 2012

© Institute of Applied Technology, 2012

ATM 1236 – Nuclear Energy Fundamentals

Module 4: Nuclear Reactors Types

Module 2: Nuclear Reactor Types Module Objectives After the completion of this module, the student will be able to:

Recognize the importance of the nuclear reactor for the operation

of the NPP.

Explain the main difference between NPP and steam power plants.

Explain the simple steam cycle.

Identify the functional requirements of the nuclear reactors.

Classify the nuclear reactors based on the type of the nuclear reaction,

the moderator material and the use.

Explain the basic principle of operation of most common nuclear

reactor types that are in use today.

Module Contents Topic Page No.

1. Introduction 3

2. The Simple Steam Cycle 3

3. Nuclear Reactors Technology 4

4. Classification of Nuclear Reactors 5

5. Typical Reactors and Current Technologies 8

6. Activities 16

7. References 18



1. Introduction

The nuclear reactor is the heart of any nuclear power plant. It provides the

thermal energy required to produce high pressure superheated steam which

is needed to generate electrical power. In normal steam power plants

thermal energy is released from burning fossil fuels which is used as a heat

source to generate electricity.

2. The Simple Steam Cycle

A schematic diagram of a simple steam power plant is shown in Fig. 3.1.

High pressure superheated steam leaves the boiler which is also referred to

as a steam generator and enters the turbine. The steam expands in the

turbine and in doing so, does work, which enables the turbine to drive the

electric generator. The low pressure steam leaves the turbine and enters

the condenser, where heat is transferred from the steam (causing it to

condense) to the cooling water. Since large quantities of water are

required, power plants are frequently located near rivers or lakes.

Fig. 3.1: Simple steam power plant.



3. Nuclear Reactors Technology

As you studied earlier the nuclear chain reaction occurs when a large fissile

atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron

and undergoes nuclear fission. The heavy nucleus splits into two or more

lighter nuclei, releasing kinetic energy, gamma radiation and free neutrons.

These products of reaction are known as fission products.

The reaction can be controlled by using neutron poisons, which absorb

excess neutrons, and neutron moderators, which reduce the velocity of fast

neutrons, thereby turning them into thermal neutrons (Fig. 3.3), which are

more likely to be absorbed by other nuclei. Increasing or decreasing the

rate of fission has a corresponding effect on the energy output of the

reactor. In summary the basic functional requirements for a thermal reactor

are:

a fuel such as Uranium -235 or Plutonium-239.

a moderator to change the fast neutrons to thermal ones.

a coolant to remove the heat.

a control system to control the number of neutrons.

a shielding system to protect equipment and people from radiation

(Fig. 3.2).

Control Rod

Reactor Shield

Reactor vessel

Moderator

Fuel Rod

Coolant Pump

Fig. 3.2: Simple schematic for a nuclear reactor.



4. Classification of Nuclear Reactors

There are several methods to classify nuclear reactors. Some of these are:

Classification by type of nuclear reaction.

Classification by moderator material.

Classification by coolant.

4.1. Classification by type of nuclear reaction

There are two types of reactors based on the nuclear reaction; these are

fission reactors and fusion reactors.

1. Fission reactors: All commercial power reactors are based on nuclear

fission. They generally use uranium and its product plutonium as nuclear

fuel. Fission reactors can be divided into two classes, depending on the

energy of the neutrons that sustain the fission chain reaction, namely

thermal reactors and fast reactors.

Thermal reactors use slowed or thermal neutrons. Almost all

current reactors are of this type. These contain neutron moderator

materials that slow neutrons until their kinetic energy approaches the

average kinetic energy of the surrounding particles (Fig. 3.3). The

probability that thermal neutrons fissioning the fissile nuclei of

uranium-235, plutonium-239, or plutonium-241 is higher compared to

the faster neutrons that originally result from fission. Using a

moderator allows the use of low-enriched uranium or even natural

uranium fuel. The moderator is often also the coolant, usually water

under high pressure to increase the boiling point.

Fast reactors use fast neutrons to cause fission in their fuel. They do

not have a neutron moderator, and use less-moderating coolants.

Maintaining a chain reaction requires the fuel to be more highly



enriched in fissile material (about 20% or more) due to the relatively

lower probability of fission versus capture by U-238. Fast reactors

have the potential to produce less nuclear waste but they are more

difficult to build and more expensive to operate. Overall, fast reactors

are less common than thermal reactors in most applications. Some

early power stations were fast reactors and also some Russian naval

propulsion units.

Fig. 3.3: Thermal and fast neutrons.

2. Fusion reactors as explained earlier in this course fusion power is still

experimental technology, generally with hydrogen as fuel.

4.2 Classification by moderator material

As explained in the previous section, moderators are used by thermal

reactors. There are four main categories based on the moderator

materials:

1. Graphite moderated reactors: These reactors tend to reduce the

kinetic energy of more fast neutrons converting them to thermal



neutrons than light water reactors. Due to the extra number of

thermal neutrons, these types can use natural uranium or un-

enriched fuel.

2. Water moderated reactors:

Light water reactors (LWRs): These reactors use ordinary water

to moderate and cool the reactors.

Heavy water reactors (HWRs): Use deuterium oxide (D2O)

which tends to increase the number of thermal neutrons and so

these types can use natural uranium or un-enriched fuel.

3. Light element moderated reactors.

Molten salt reactors (MSRs): Use a light elements such as

lithium or beryllium as a moderator.

Liquid metal cooled reactors: Use a coolant that is a mixture of

Lead and Bismuth and may use BeO as a moderator.

4. Organically moderated reactors (OMRs): Use biphenyl and terphenyl

as moderator and coolant.

4.3. Classification by use

The following are the most common uses of nuclear reactors: 1. Electricity

Nuclear power plants

2. Propulsion

Nuclear marine propulsion

Various proposed forms of rocket propulsion

3. Heat

Desalination plants

Heat for domestic and industrial heating

Hydrogen production for use in a hydrogen economy



4. Research reactors: These reactors are used for research and training,

materials testing, or the production of radioisotopes for medicine and

industry. These are much smaller than power reactors or those

propelling ships, and many are on university campuses. Sharjah

University has one of these.

5. Production reactors for transmutation of elements such the breeder

reactors and reactors used to produce weapon grade plutonium.

5. Typical Reactors and Current Technologies

In this section we will discuss some of the typical reactors used around the

globe for electrical power generation.

5.1. Boiling water reactors (BWR)

In this reactor the enriched uranium oxide UO2 is used as the fuel and light

water is used as the coolant and moderator. This is a thermal reactor (Fig.

3.4). The water is circulated by a pump and the water boils in the reactor

vessel itself. The steam produced is fed directly to turbine. In BWR, the

steam is generated in the core itself.

Fig. 3.4: Boiling water reactor (BWR).

The reactor vessel has to be strong and is enclosed in concrete containment

vessel to prevent hazard from the failure of the pressurized circuit.



The exhaust steam from turbine is condensed and the condensate is sent

back to the reactor core through a feed pump. Another pump is used for re-

circulating the coolant in the reactor vessel before converting to steam.

5.2. Pressurized water reactors (PWR)

PWRs keep water under pressure so that it gets heat, but does not boil (Fig.

3.5). Water from the reactor and water in the steam generator that is

turned into steam never mix. Because of this, most of the radioactivity

stays in the reactor area (primary loop).

Secondary Loop

Primary Loop

Fig. 3.5: Pressurized water reactor (PWR).

A primary characteristic of PWRs is a pressurizer, a specialized pressure

vessel (Fig. 3.6). During normal operation, a pressurizer is partially filled

with water, and a steam bubble is maintained above it by heating the water

with submerged heaters. During normal operation, the pressurizer is

connected to the primary reactor pressure vessel and the pressurizer

"bubble" provides an expansion space for changes in water volume in the



reactor. This arrangement also provides a means of pressure control for the

reactor by increasing or decreasing the steam pressure in the pressurizer

using the pressurizer heaters.

The PWRs are the majority of current reactors, and are generally considered

the safest and most reliable technology currently used in large scale

deployment.

Pressurizer

Steam generator

Fig. 3.6: a) Pressurizer. b) Installing the pressurizer in a nuclear reactor.

5.3. Pressurized heavy water reactors (PHWR)-CANDU

These reactors are pressurized water reactors. These reactors are thermal

reactors and fueled with natural uranium. They use heavy water

(Deuterium; D2O) as a moderator and they also use light water or heavy

water as a coolant.

Instead of using a single large pressure vessel as in a PWR, the fuel is

contained in hundreds of pressure tubes.. PHWRs can be refueled while at

full power, which makes them very efficient in their use of uranium. PHWR

is also known as CANDU (CANada Deuterium-Uranium) because it is a

Canadian design. Fig. 3.7 shows the main components of this type of

reactors. These components are: 1. Fuel bundle 2. Calandria (reactor core)

3. Control rods 4. Pressurizer 5. Steam generator 6. Light water pump

7. Heavy water pump 8. Moderator (D2O) 9. Pressure tubes 10. Steam

11. Cold water from condenser 12. Concrete shielding.



Fig. 3.7: Pressurized heavy water reactor (CANDU) schematic.

5.4. Advanced power reactors (APR1400).

In early 2010, South Korea won its first export order; four APR-1400

reactors for the United Arab Emirates. The APR1400 is an advanced version

of the OPR1000.

The power generation capacity of the APR1400 is 1400 MW. APR1400 is

PWR and classified as thermal reactor that uses enriched U-235 as a fuel.

The design life of the major components such as the reactor vessel and the

steam generator has been extended from 40 years to 60 years compared to

the earlier version (OPR1000).

The reactor which consists of a vertical cylindrical shell, hemispherical lower

head and hemispherical upper head manufactured by forging stainless steel



lining. Fig. 3.8 shows the main components of the APR1400.

Steam generators

Pressurizer

Cooling pumps

Reactor vessel

Fig. 3.8: Advance Power Reactor (APR 1400) schematic.

5.5. Gas-cooled reactor (GCR)

GCRs are nuclear reactors that use graphite as a neutron moderator and

carbon dioxide or helium as coolant (Fig. 3.9). There are many other types

of reactor cooled by gas, however the term GCRs and to a lesser extent

gas- cooled reactors are particularly used to refer to this type of reactor.

The GCR uses natural uranium as fuel, which enable the countries that

developed them to fabricate their own fuel without relying on other

countries for supplies of enriched uranium. GCRs are designed for on-load

refueling. GCRs are nearly exclusively operating in Great Britain (Magnox

reactors).



Fig. 3.9: Gas-cooled reactor schematic.

5.6. Advanced Gas-cooled Reactor (AGR)

These are the second generation of British gas-cooled reactors , using

graphite moderator and carbon dioxide as coolant (Fig. 3.10).

Fig. 3.10: Advanced Gas-cooled Reactor (AGR) schematic.



The fuel is uranium oxide pellets, enriched to 2.5-3.5%, in stainless steel

tubes. The carbon dioxide circulates through the core, reaching 650°C and

then past steam generator tubes outside it, but still inside the concrete and

steel pressure vessel. Control rods penetrate the moderator and a

secondary shutdown system involves injecting nitrogen to the coolant.

5.7. Fast breeder reactor (FBR)

FBRs have been designed not only to produce electricity, but also to

generate (breed) fuel, namely plutonium out of uranium. They use a

plutonium (Pu) fuel rather than uranium (Fig. 3.11). The Pu is surrounded

by rods of U-238 which absorb neutrons and are transformed into Pu-239.

The principal reactor cooling system design of a FBR is similar to a PWR.

The major difference is that the coolant in a FBR is liquid sodium instead of

water. There are 2 sodium circuits, where heat transfer is generated by a

heat exchanger. The secondary sodium circuits transfer the heat to a steam

generator. The function of the steam generator is similar to its function in

other types of reactors.

Fig. 3.11: Fast Breeder Reactor (FBR) schematic.



The FBR fuel is plutonium which needs fast neutrons to generate fission,

therefore water cannot be used as a coolant, because of its moderating

function. The lack of a moderator to slow neutrons gives the word fast to

the name.

Table 1.1 shows the most common reactor types used all over the globe. It

shows also the main countries, the number of reactors and the type of fuel,

coolant and moderator used in each type. Table 4.1 shows that the most

dominant type amongst all types is the pressurized water reactor (PWR).

Table 4.1: Reactor types and numbers.

Reactor Type Main Countries Number Fuel Coolant Moderator

Pressurized Water

Reactor (PWR)

US, France, Japan,

Russia, China 265 Enriched UO2 Water Water

Boiling Water

Reactor (BWR)

US, Japan,

Sweden 94 Enriched UO2 Water Water

Pressurized Heavy

Water Reactor

'CANDU' (PHWR)

Canada 44 Natural UO2 Heavy

water

Heavy

water

Gas-cooled

Reactor

(GCR)/AGR

UK 18 Natural U/

Enriched UO2 CO2 Graphite

Light Water

Graphite Reactor Russia 12 Enriched UO2 Water Graphite

Fast Neutron

Reactor (FBR)

Japan, France,

Russia 4 PuO2 and UO2

Liquid

sodium None

Other types Russia 4 Enriched UO2 Water Graphite

TOTAL 441



6. Activities

6.1. Activity 1

1. Identify the NPP parts by writing the number of the correct power plant

part in the blank.

1. Reactor 5. Steam generator 9. Primary water loop

2. Control rods 6. Turbine–generator 10.Secondary water loop

3. Cooling water 7. Transmission lines 11. Nuclear fuel

4. Containment building 8. Condenser 12. Pressurizer

13. Cooling tower

2. Colour the separate loops using a different colour for each loop. Use the

following symbols to show what is in the loop or part of the loop.



6.2. Activity 2

1. Identify the NPP parts by writing the number of the correct power plant

part in the blank.

1. Reactor 4. Condenser 7. Nuclear fuel

2. Control rods 5. Turbine–generator 8. Containment building

3. Cooling water loop 6. Transmission lines

2. Colour the separate loops using a different colour for each loop. Use the

following symbols to show what is in the loop or part of the loop.

6.3. Activity 3

Build a 3D physical model of BWR and/or PWR. The material that will be

used to build the model is left to you to decide. Think of your model as a

part that will be integrated in a comprehensive model illustrating a

complete layout of a NPP.



7. References

1. Why Science Matters, Harnessing the Sun’s Energy, Andrew Solway,

Heinemann Library.

2. Nuclear Energy (Fuelling the Future), Chris Oxlade and Elizabeth

Raum (Heinmann Library, 2008).

3. Future Energy, Improved, Sustainable and Clean Option For Our

Planet, Edited by Trevor M. Letcher, Elsevier.

4. Nuclear Safety, Gianni Petrangeli, Elsevier.

5. http://www.bwr-pr.com/

6. http://en.wikipedia.org/wiki/Boiling_water_reactor

7. http://www.nuclearfaq.ca/cnf_sectionA.htm

8. http://commons.wikimedia.org/wiki/Category:Schemata_of_pressuriz

ed_water_reactor

9. http://www.nuc.umr.edu/~ans/poor.html

10. http://nuceng.mcmaster.ca/refer/facts.htm

11. http://www.apr1400.com/

12. http://www.solcomhouse.com/nuclearpowerplants.htm

13. http://www.ecology.at/nni/index.php?p=type&t=gcr

14. www.nukeworker.com

15. http://www.ustudy.in

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